1. Field of the Invention
The present invention relates to an air driven motor adapted for use with a downhole drill hammer, and more particularly to a motor which allows such a hammer to be used for directional drilling.
2. Description of the Prior Art
Directional drilling is a technique whereby a bore is cut into the earth at some angle away from the vertical. The use of this technique is warranted by both economics and physical impossibilities in attempting to reach a target some distance below the surface of the earth. Oil, gas and other natural resources are not always conveniently located so that a drill can be erected immediately above a reserve and cut straight down into it. Most often, directional drilling is utilized to avoid surface irregularities, such as river beds or mountains; to secure the most advantageous angle of attack when passing through a fault zone; avoiding salt dome deposits above the target; and to allow several bores to be drilled from a single location to minimize expense.
In all applications, the bore is begun in a vertical direction, normal to the surface of the earth. At some predetermined time, the angle of the bore is changed and directional drilling begins. This is carefully planned to select the optimum location for the initial vertical bore, and the depth at which the change in angle begins. Also planned is the degree of change and the rate of change in angle. A change in angle that is too rapid will not have the accuracy of a long slow curve towards the target. These parameters are based on surface conditions and changes in the rock structure between the surface and the target.
The problem as presented to the inventor is generally one of accuracy and penetration rate, especially in deviated and faulted rock formations and hard rock. Examples of hard rock are limestone, sandstone, granite, and metamorphic and igneous rock in general. The term hard rock is used to describe structures having a compressive strength generally greater than 20,000 psi. Once the directional drilling begins, the bore must be carefully monitored to determine the angle of attack. The crucial parameters are force on the bit and the rotational speed of the drill bit. Higher force and faster speed allow for faster drilling, but accuracy suffers accordingly if drilling is done utilizing conventional techniques. Consequently, the expense of directional drilling is much higher in terms of time per distance drilled than vertical drilling.
The primary problem with the current directional drilling techniques is the available equipment. Although, there are many different types of drill motors and bits currently being used for a wide range of drilling needs, two types of drills are generally used and are of particular interest in downhole drilling. One type, called a valveless downhole drill or drill hammer, is exemplified by Ingersol Rand's DHD-380. This drill hammer is currently used exclusively in vertical drilling. The state of the art does not permit the use of this drill in directional drilling for reasons explained later. Its advantages in downhole vertical drilling are its simple valveless design which requires less maintenance, and its higher power and faster drilling ability. It drills by utilizing both a percussive impact on the rock and a rotary twisting motion. The percussive impact is produced in an oscillating piston-like manner, driven by compressed air from the surface, while the entire hammer body is rotated on a rotational drill pipe or string to produce a rotary motion at the bit. It is this rotary motion which prevents the drill from being used in directional drilling, since the drill string cannot rotate once a non-vertical bore is begun.
The second type of drill discussed here is the downhole motor utilizing the "tri-cone" bit. This model is exemplified by the Norton Christiensen Navi-Drill Mach 1. This type of drill is currently the only model which can be utilized in directional downhole drilling. It can also be utilized in vertical drilling. The motor operates by forcing a liquid mud mixture down the drill pipe which turns a rotor or stator inside the drill casing. This rotor in turn rotates the three cone-shaped bits at the tip of the drill motor. The liquid also serves to cool and lubricate the rotor apparatus. This tool does not have the same power or speed in drilling hard rock as the hammer-type drill, and also requires more maintenance in the form of bit changes and overheating wear in high force drilling applications.
At the same time, the use of mud or slurry to drive a drilling apparatus is inefficient when compared to the use of compressed air, due to increased maintenance required, labor times, and cost. Increased force on the bit or higher pressures of fluid movement increase the efficiency of the motor and bit themselves, but at the same time increase deterioration of both components. Increased wear and deterioration also causes more maintenance time and less time actually drilling. Air cannot be efficiently utilized in the current downhole drill motors used in directional drilling, because the air does not have the force necessary to turn the bit at an efficient speed, nor does it have the cooling ability of the liquid mud and slurry.
The use of the hammer type drill alleviates many of these problems. The use of mud as a drive means is abandoned, and the efficiency of the drill allows for less drilling time to penetrate a similar distance than with a tri-cone drill. This results in less maintenance time as well as less deterioration of the equipment.
The main reason for the use of the hammer when drilling in a deviated or faulted formation, however, is the increased trajectory accuracy and penetration rate. The hammer requires much less force from the drill string compared to a tri-cone drill cutting at the same speed in distance per unit time. Similarly, the rotational speed of the bit is much less for the impact-type hammer. These two parameters are the key to accurate drilling. By reducing the force and speed of the drill, a trajectory may be more carefully traced to the target, while quickly attaining the goal utilizing the hammer drill.